linux-kernel - Re: v5.9-rc3-rt3 boot time networking lockdep splat

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Date:   Tue, 8 Sep 2020 17:06:12 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Mike Galbraith <efault@....de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: v5.9-rc3-rt3 boot time networking lockdep splat

On 2020-09-08 16:56:20 [+0200], Mike Galbraith wrote:
> On Tue, 2020-09-08 at 14:19 +0200, Sebastian Andrzej Siewior wrote:
> >
> > This has nothing to do with the bridge but with the fact that you use a
> > non standard queue class (something else than pfifo_fast).
> 
> That must be SUSE, I don't muck about in network land.  I downloaded a
> whole library of RFCs decades ago, but turns out that one of those is
> all the bedtime story you'll ever need.  Huge waste of bandwidth :)

I see.
This should cure it:

Subject: [PATCH] net: Properly annotate the try-lock for the seqlock

In patch
   ("net/Qdisc: use a seqlock instead seqcount")

the seqcount has been replaced with a seqlock to allow to reader to
boost the preempted writer.
The try_write_seqlock() acquired the lock with a try-lock but the
seqcount annotation was "lock".

Opencode write_seqcount_t_begin() and use the try-lock annotation for
lockdep.

Reported-by: Mike Galbraith <efault@....de>
Cc: stable-rt@...r.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 include/net/net_seq_lock.h |  9 ---------
 include/net/sch_generic.h  | 10 +++++++++-
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/include/net/net_seq_lock.h b/include/net/net_seq_lock.h
index 95a497a72e511..67710bace7418 100644
--- a/include/net/net_seq_lock.h
+++ b/include/net/net_seq_lock.h
@@ -6,15 +6,6 @@
 # define net_seq_begin(__r)		read_seqbegin(__r)
 # define net_seq_retry(__r, __s)	read_seqretry(__r, __s)
 
-static inline int try_write_seqlock(seqlock_t *sl)
-{
-	if (spin_trylock(&sl->lock)) {
-		write_seqcount_begin(&sl->seqcount);
-		return 1;
-	}
-	return 0;
-}
-
 #else
 # define net_seqlock_t			seqcount_t
 # define net_seq_begin(__r)		read_seqcount_begin(__r)
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 796ac453d9519..40be4443b6bdb 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -168,8 +168,16 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 		return false;
 	}
 #ifdef CONFIG_PREEMPT_RT
-	if (try_write_seqlock(&qdisc->running))
+	if (spin_trylock(&qdisc->running.lock)) {
+		seqcount_t *s = &qdisc->running.seqcount.seqcount;
+		/*
+		 * Variant of write_seqcount_t_begin() telling lockdep that a
+		 * trylock was attempted.
+		 */
+		raw_write_seqcount_t_begin(s);
+		seqcount_acquire(&s->dep_map, 0, 1, _RET_IP_);
 		return true;
+	}
 	return false;
 #else
 	/* Variant of write_seqcount_begin() telling lockdep a trylock
-- 
2.28.0


> 	-Mike

Sebastian